The present invention relates to a material for forming an insulating film with low dielectric constant, an insulating film with low dielectric constant, a method for forming an insulating film with low dielectric constant and a semiconductor device including an insulating film with low dielectric constant.
Recently, a multilayer interconnect structure including an insulating film with low dielectric constant is necessary for realizing refinement, a high-speed operation and a low power consuming operation of a semiconductor device.
A conventional insulating film used in a multilayer interconnect structure is, for example, a silicon oxide film having a dielectric constant of approximately 4.2 or a silicon oxide film doped with fluorine having a dielectric constant of approximately 3.7. Also, in order to further lower the low dielectric constant, an organic component-containing silicon oxide film doped with a methyl group (CHF3) is recently under examination.
It is, however, very difficult to lower the dielectric constant of an organic component-containing silicon oxide film below 2.5, and therefore, an insulating film having pores, namely, what is called a porous film, is necessary.
Now, conventional technique for a porous film will be described.
First, a first conventional example and a second conventional example disclosed in Japanese Laid-Open Patent Publication No. 2001-294815 will be described.
In the first conventional example, a porous film is formed by baking a thin film made from a solution including a silicon resin and an organic solvent. In this example, open pores are randomly formed in portions where the organic solvent has been vaporized in baking the thin film. In this case, the organic solvent has a function as a solvent as well as a function to form the pores. In general, a spin coating method is employed for forming the thin film by applying the solution on a substrate, and a hot plate and a furnace (electric furnace) are used for baking the thin film.
In the second example, a porous film is formed by baking a thin film made from a solution including not only a silicon resin and an organic solvent but also a porogen of an organic substance. In this example, not only open pores but also closed pores can be formed through selection of the porogen. In this case, the porogen is naturally vaporized to disappear from the resultant film.
Next, a third conventional example disclosed in Japanese Laid-Open Patent Publication No. 8-181133 will be described.
A porous film of the third conventional example has conceptually the most general structure and is formed by using a solution as shown in FIG. 9. Specifically, as shown in FIG. 9, a solution in which a silicon resin 102, a porogen 103 and a solvent 104 are mixed is contained in a vessel 101.
In the third conventional example, which is disclosed in Japanese Laid-Open Patent Publication No. 8-181133, a porous film is formed by baking a thin film made from a solution including a fullerene such as C60 or C70, a silicon resin and an organic solvent. In this case, a hollow portion of the fullerene becomes a pore of the porous film.
As the silicon resin used in the first, second and third conventional examples, an organic silicon resin such as methylsilsesquioxane capable of lowering the dielectric constant as compared with an inorganic silicon resin is used.
Now, an exemplified conventional method for forming a thin film from a solution will be described with reference to FIGS. 10A through 10F. In general, a substrate on which a thin film has been formed by the spin coating method is baked with a hot plate or an electric furnace.
First, as shown in FIG. 10A, a semiconductor wafer 112 is placed on a spindle 111 connected to a rotation mechanism, and thereafter, an appropriate amount of solution 114 used for forming a porous film is dropped on the semiconductor wafer 112 from a solution supply tube 113.
Next, as shown in FIG. 10B, the spindle 111 is rotated so as to rotate the semiconductor wafer 112, and thus, the solution 114 is spread so as to form a thin film 115.
Then, as shown in FIG. 10C, the semiconductor wafer 112 on which the thin film 115 has been formed is placed on and annealed with a hot plate 116 so as to vaporize the solvent. This procedure is generally designated as pre-bake and is performed at a temperature of approximately 100° C. for approximately 1 through 3 minutes.
Next, as shown in FIG. 10D, the semiconductor wafer 112 is placed on a hot plate 117 to be annealed at a temperature of approximately 200° C. for 1 through 3 minutes. This procedure is generally designated as soft bake.
Thereafter, as shown in FIG. 10E, the resultant semiconductor wafer 112 is placed in an electric furnace 118, and then, the temperature of the electric furnace 118 is increased to approximately 400° C. through 450° C., so that annealing can be performed at the highest set temperature for approximately 1 hour. This procedure is generally designated as hard bake, and when this procedure is completed, a porous film 115A is formed on the semiconductor wafer 112. The hard bake can be performed by using a hot plate. Also, in using some solution, annealing is preferably performed, between the soft bake and the hard bake, with a hot plate at an intermediate temperature between the temperatures of the soft bake and the hard bake for approximately 1 through 3 minutes.
FIG. 10F is an enlarged view of a portion surrounded with an alternate long and short dash line in FIG. 10E. As is understood from FIG. 10F, pores 119 (white portions in the drawing) are formed in the porous film 115A formed on the semiconductor wafer 112.
The mechanical strength of the porous film 115A obtained through nano-indentation evaluation is at most approximately 5 GPa in the Young's modulus. With respect to insulating films that are currently actually used in semiconductor devices, the modulus of a silicon oxide film is approximately 78 GPa, the modulus of a fluorine-containing silicon oxide film is approximately 63 GPa and the modulus of an organic component-containing silicon oxide film is approximately 10 GPa. Thus, the mechanical strength of the porous film 115A is smaller than that of any other insulating film used in a multilayer interconnect structure of a current semiconductor device, and accordingly, a porous film with larger mechanical strength is desired to be developed.
FIG. 11 shows the cross-sectional structure obtained in bonding a wire to a semiconductor device that has a three-layer interconnect structure and uses a conventional porous film as an insulating film. In FIG. 11, a reference numeral 120 denotes a semiconductor wafer, a reference numeral 121 denotes a porous film, reference numerals 122, 124 and 126 denote metal interconnects, reference numerals 123, 125, 126 and 128 denote via plugs and a reference numeral 129 denotes a pad to be connected to an external interconnect.
As shown in FIG. 11, when a wire 130 is bonded to the upper face of the pad 129, a crack is caused in the pad 129 and the multilayer interconnects.
The mechanical strength of the porous film 115A is necessary for retaining multilayered interconnects stacked for forming a multilayer structure as well as in bonding for mounting a chip of a semiconductor device in a package as described above. In the case where an organic component-containing silicon oxide film is used as an insulating film, the mechanical strength is at a level of the very limit of breakdown obtained in employing the current bonding technique, and although the bonding technique is expected to be further developed in the future, development of a porous film with large mechanical strength is of urgent necessity.
In the first and second conventional examples, the open pores are randomly formed. Therefore, in order to realize an insulating film with a dielectric constant k of 2.2 through 2.3, the Young's modulus of approximately 5 GPa or less in the nano-indentation evaluation can be attained at most. This mechanical strength depends upon the method for forming the film in the first or second example. Specifically, the porogen and the solvent are not present but the silicon resin alone is present in the porous film after the bake, and therefore, the mechanical strength of the porous film depends upon the original strength of the silicon resin and the porosity (a ratio occupied by pores in a unit volume). In the first or second conventional example, when the dielectric constant is to be further lowered, the porosity is increased, which further lowers the mechanical strength.
In the third conventional example, although the fullerene remains in the porous film after the bake, the mechanical strength basically depends upon the strength of the silicon resin including the fullerene and hence is at the same level as that attained in the first or second conventional example. Also, when the content of the fullerene exceeds approximately 30 wt %, the fullerenes are connected to each other, and therefore, the mechanical strength is further lowered.
As described so far, a practically usable rigid film cannot be obtained by any of the conventional methods for forming a porous film because there is a limit in the mechanical strength of the structure itself of the porous film of a silicon resin.
Also, a conventional porous film can attain merely mechanical strength much lower than the mechanical strength necessary for a semiconductor device, and when the dielectric constant of the porous film is to be lowered, the mechanical strength is disadvantageously lowered.
As a result, in the case where a conventional porous film is actually used in a multilayer interconnect structure of a semiconductor device, there arise a problem that a semiconductor device with sufficient strength cannot be fabricated and a problem that even when a semiconductor chip can be fabricated, the semiconductor device cannot be completed because it is broken in mounting the chip in a package.